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744 peculiar and characteristic shape. They are oval, each egg on an average measuring about 0·16 mm. in length by 0·06 mm. in breadth, and one end of the ovum is provided with a short, stout, and very definite spine. (Plate XIII., Fig. 1.)

The exact nature of the process by which the ova leave the body of the human host has not been satisfactorily explained. Apparently the female worm migrates from time to time from the larger veins to their smaller radicles, and in these deposits her ova. The walls of the bladder are the favourite situations for this purpose. Afterwards the eggs are somehow carried, possibly aided by the spine with which they are provided, towards the surface of the mucous membrane, and then, falling into the bladder, are voided with the urine, a certain amount of blood escaping at the same time.

The free larva (Fig. 132).—In newly voided urine the ovum

presents a somewhat brownish appearance, and generally contains a ciliated larva (miracidium). After a time the larva may escape through a longitudinal rupture in the shell. It then swims about, but, unless supplied with fresh water, soon perishes. If the urine be freely diluted with water, the larva not only escapes more quickly from the shell but also continues to live, swimming and gyrating very actively, for a considerable time. While swimming, the body of the little animal undergoes many changes of shape. For the most part, when advancing, it is oblong, tapering somewhat posteriorly; when more stationary it tends to assume a spherical form. It moves by means of the cilia which, with the exception of the minute papillary beak, thickly cover the entire body. On carefully examining the larva, a canal may be traced from the beak into what looks like a rudimentary stomach; on both sides of this, two much smaller gland-like organs can be seen, from each of which a delicate tube passes forwards and opens, apparently, somewhere in the neighbourhood of the beak.